Lipid composition of gallbladder bile, bile acid pool size and sex hormone levels are being determined in Pima Indians below 21 years of age to determine the age at which Pima Indian bile becomes lithogenic. These data will indicate the approximate duration of bile lithogenicity prior to the appearance of cholesterol gallstones and indicate the period of life at which prophylactic measures against gallstone formation may be taken. Bile is obtained by duodenal siphonage after stimulation of gallbladder contraction and bile acid pool size is measured by isotope dilution using deuterated chenodeoxycholic acid. Changes in bile lipids will be correlated with age, sex, level of sex hormones, pubertal development, body size, and plasma lipoprotein patterns.